Can you go into Section Editor and do some screen shots on what the corridor looks like in a good cross section location and then in one of these bad cross section locations? That might help trouble shoot how the calculation is going wrong.
One idea that it may be is, is the outer boundary of your target surface always beyond the limits of your Work Area? The error you are getting is in regards to either P4 or P7 (your two intersection points) not being able to be calculated, which would therefore mean that they do not have AL4 or AL2 present for them to calculate from in the error locations.
Kati Mercier, P.E. | LinkedIn | AutoCAD Civil 3D Certified Professional
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Co-author of "Mastering AutoCAD Civil 3D 2013"
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